OA12415A

OA12415A - Fluid mixing system.

Info

Publication number: OA12415A
Application number: OA1200300147A
Authority: OA
Inventors: Joeel Rondeau; Pierre Vigneaux
Original assignee: Sofitech Nv
Priority date: 2000-11-29
Filing date: 2001-10-04
Publication date: 2006-04-18
Also published as: NO329657B1; ATE277271T1; AR031355A1; EA200300616A1; US20040100858A1; AU2002223029B2; CA2429292A1; US6491421B2; DE60105852T2; US6786629B2; US7226203B2; NO20032446L; CN1484730A; CA2429292C; WO2002044517A1; EG23123A; DE60105852T8; BR0115636A; CN1256500C; DE60105852D1

Abstract

A method for continuously mixing a borehole fluid such as cement includes using a measurement of the solid fraction of a cement slurry as it is being mixed to determine the ratio of the solid and liquid components to be added to the slurry. A system for mixing the includes a liquid material (water) supply including a flow meter; a solid material (cement) supply; a mixer which receives the liquid and solid materials and includes an output for delivering materials from the mixer to a delivery system; a device for measuring the amount of material in the mixer; and a flow meter in the output; wherein measurements from the flow meters and the device for measuring the amount of material in the mixer are used to control the amount of solid and/or liquid material added to the mixer.

Description

1 012415

The présent invention relates to a System for mixing fluids containing solid and liquid materialssuch as cernent. In particular the invention provides a System for the continuons mixing ofcements or other fluids used in the drilling, completion or stimulation of boreholes such as oil orgas wells.

When a well such as an oil or gas well has been drilled, it is often desired to isolate the variousproducing zones form each other or from the well itself in order to stabilise the well or preventfluid communication between the zones or shut off unwanted fluid production such as water.This isolation is typically achieved by installing a tubular casing in the well and filling theannulus between the outside of the casing and the wall of the well (the formation) with cernent.The cernent is usually placed in the annulus by pumping a slurry of the cernent down the casingsuch that it exits at the bottom of the well and passes back up the outside of the casing to fill theannulus. While it is possible to mix the cernent as a batch prior to pumping into the well, it hasbecome désirable to effect continuous mixing of the cernent slurry at the surface just prior topumping into the well. This has been found to provide better control of cernent properties andmore efficient use of materials.

The cernent slurries used in such operations comprise a mixture of dry and liquid materials. Theliquid phase is typically water and so is readily available and cheap. The solid materials defmethe slurry and cernent properties when added to the water and mixed, the amount of solidmaterials in the slurry being important. Since the liquid phase is constant, the amount of solidmaterial added is usually monitored by measuring the density of the slurry and maintaining thisat the desired level by controlling the amount of the solid material being added. Figure 1 shows aschematic diagram of a prior art mixing System. In the System of Figure 1, mix water is pumpedfrom a feed supply 10 via a pump 12 to a mixer 14 which feeds into a mixing tub 16. The feedsupply 10 comprises a pair of displacement tanks 11,11’ each with separate outlets connected toa valve 13 which in tum feeds the pump 12. Two methods are commonly used to détermine theamount of water supplied: 2 012445 1. Proximity switches installée! on the shaft of the pump 12 count a number of puises perrotation. Each puise corresponds to a displacement volume. This method is sensitive to pumpefficiency. 2. Displacement volume is measured by counting the number of tanks pumped down-hole. Thismeasurement method is sensitive to human error in level reading, switching from on tank toanother and tank exact capacity. Even more an error in the number of tanks counted can hâvemany conséquences (over displacement can resuit in wet shoe, under displacement can resuit inno pressure bump or cernent left in the casing).

Solid materials are delivered to the mixer 14 from a surge can 18 or directly from a cernent silovia a flow control valve 20 and are carried into the mixing tub 16 with the mix water. Thecontents of the mixing tub 16 are recirculated through a recirculation pipe 22 and pump 24 to themixer 14. The recirculation pipe 22 also includes a densitometer 26 which provides ameasurement of the density of the slurry in the mixing tub 16. An output 28 is provided forslurry to be fed from the mixing tub 16 to further pumps (not shown) for pumping into the well.Control of the slurry mixture is achieved by controlling the density in the mixing tub 16 asprovided by the densitometer 26 by addition of solid material so stay at a predetermined level forthe slurry desired to be pumped. The densitometer 26 is typically a non-radioactive device suchas a Coriolis meter.

While this system is effective for slurries using materials of much higher density than water, it isnot effective for slurries using low density solid materials, especially when the density of thesolids is close to that of water. In such cases, a density measurement is not sensitive enough tocontrol the amounts of solid material added to the necessary accuracy.

The présent invention seeks to provide a mixing System which avoid the problem of densitymeasurement described above.

In its broadest aspect, the présent invention comprises using a measurement of the solid fractionof a fluid as it is being mixed to détermine the ratio of the solid and liquid components added tothe slurry. 3 012415

The invention is particularly applicable to the mixing of borehole cernent slurries, in which case,solids fraction is determined as (slurry vol - water vol)/slurry vol. An alternative but relatedparameter is porosity, determined as water vol/slurry vol (porosity + solids fraction =1). A System for mixing cernent in accordance with the invention comprises a liquid (water) supply5 including a device for measuring the amount of liquid supplied; a solid material supply; a mixerwhich receives the liquid and solid materials and includes an output for delivering materials fromthe mixer to a delivery System; a device for measuring the amount of material in the mixer; and aflow meter in the output; wherein measurements from the flow meters and the device formeasuring the amount of material in the mixer are used to control the amount of solid material 10 added to the mixer.

The flow meters can be mass flow meters or volumétrie flow meters. Any suitable form of metercan be used, for example Coriolis meters or electromagnetic meters.

The mixer will typically include a tank or tub, in which case the device for measuring the amountof material in the mixer can be a level sensor. Such a level sensor is preferably a time domain 15 reflectometry- or radar-type device although acoustic or float devices can also be used. It ispreferred to mount such a device in an arrangement for damping transient fluctuations in the tanklevel, for example in an arrangement of concentric slotted tubes. An alternative or additionalform of sensor can be a load cell which can be used to indicate the weight of the tank, or apressure sensor. 20 The device for measuring the amount of liquid supplied can be a flow meter or a level sensor ofthe types described above. When the liquid supply includes one or more displacement tanks, alevel sensor is preferred

Where the mixer includes some form of recirculation of the slurry through the tank, it is important that the output flow meter is downstream of this recirculation. 4 012415

Where the solid materials comprise cernent and other solid additives added separately to the mixer, separate flow meters can also be provided for each separate supply of additives.

In its simplest form, the measurement of solid fraction is used as a guide for the operator to addsolids, particularly cernent, to the slurry as it is mixed. In more advanced versions, thecalculation of solids fraction is used to control the addition of solids directly by means of anautomatic control System.

The invention also provides an improved method for calculating displacement volume from aSystem comprising at least one displacement tank, comprising measuring the level of liquid inthe tanks over time and calculating displacement as: Σ (V(hln) - V(h2n)) where: V(h) is the exact volume of the tank at level (h); hln is the start level of the n th displaced tank volume; and h2n is the stop level of the n th displaced tank volume.

Examples of the présent invention will now be described with reference to the accompanyingdrawing, in which:

Figure 1 shows a prior art mixing System;

Figure 2 shows a mixing System according to a first embodiment of the invention;

Figure 3 shows the components of a tank level sensor;

Figure 4 shows the components of the level sensor assembled;

Figure 5 shows a schematic of the tank level measurement; and

Figure 6 shows a mixing System according to a second embodiment of the invention.

The system shown in Figure 2 is used for the continuous mixing of cernent for oil well cementing operations and comprises a supply of mix water 100 feeding, via a pump 102 and a flow meter 104 to a mixing system 106. 5 0124 15

The supply of mix water comprises a pair of displacement tanks 101, each having a separate output connected to a valve 103 whiçh supplies the pump 102. Level sensors 105 are included in each displacement tank 101 for determining the amount of water supplied to the pump 102. In another version (not shown), the level sensors are omitted. The amount of water supplied is determined in the manner described below.

The mixing System 106 also receives solid materials from a surge can 108 (or altemativelydirectly from a surge can) which are admitted through a valve 110. The mixed solid and liquidmaterials are delivered through a feed pipe 112 to a mixing tub 114. The mixing tub 114 has afirst outlet 116 connected to a recirculation pump 118 which feeds the slurry drawn from the tub114 back into the mixing System. The tub 114 is provided with a level sensor 120 and/or a loadsensor 122 to provide an indication of the tank contents and any change in contents over time. Asecond output 124 is provided from the tub 114 which leads, via a second pump 126 and asecond flow meter 128 to the pumping System from which it is delivered to the well (not shown).An alternative method of delivery (shown in dashed line in Figure 2) has an output 124’ takenfrom the recirculation line via a flow meter 128’ to the well. Other arrangements are alsopossible. The pumps 102, 118, 126 are of the usual type found in well cementing Systems, forexample centrifugal pumps. Likewise, the flow meters 104, 128’ are conventional, for exampleCoriolis meters such as those that hâve been used as densitometers in previous applications.Different types of pumps and meters each hâve advantages and disadvantages that are wellknown in the art and can be selected according to requirements.

Figures 3-5 show details of the level sensors used in the displacement tanks and tub and themanner of installation. The sensor comprises a Krohne radar sensor 200, a stainless Steel rod 202,an inner slotted sleeve 204 and an outer slotted sleeve 206. The rod 202 is screwed onto thesensor 200 and the inner sleeve 204 mounted over the rod 202 and attached to a flange on thesensor 200. The outer sleeve 206 is mounted over the inner sleeve 204 to which it is attached.

For use in the displacement tanks, each displacement tank receives a level sensor. This sensorgives an accurate measurement of the liquid level in the tank. The exact volume versus level is 6 °1?415 required to calculate the displaced volume. In case the tank cross section profile is not accuratelyknown a so-called tank calibration is performed. A water meter equipped with a digital outputmeasures the exact displacement tank volume versus tank level. This operation is performed onlyonce for each tank. To supply water to the System, the valve 103 is operated to allow water toflow from one or other tank to the pump 102. When a tank discharge valve is opened, a devicesuch as an end switch, pressure switched or any other appropriate device is used to begincalculation of the displacement volume. Displacement volume is then computed as: Σ (V(hln) - V(h2n))

Where: V(h) is the exact volume of the tank at level (h) hln start level of the n th displaced tank volume h2n stop level of the n th displaced tank volume

When the level in the tank in use becomes low, the supply is switched to the other tank.

Switching operation from one tank to another can either be manual or automated and when onetank is emptying the other one is filled up for further use. Since the level sensors can be used togive an instantaneous measurement of the amount of water provided to the System, it is possibleto confirm the data provided by the flow meter 104, or even to replace the need for this flowmeter completely. When the flow meter is présent, it is not essential to hâve the level sensors inthe displacement tanks.

This method of determining the displacement volume can be applied to other forms of cementingoperation than the ones described here, and has the advantage that it is relatively insensitive topump efficiency or operator error as found in the previous Systems.

For use in the mixing tub, the sensor arrangement is installed in the mixing tub 114 in thevertical position and in a location where the slurry is renewed as the mixing occurs, to avoidlocation in a dead zone where cernent might set. The sensor provides a measurement of thedifférence between the length of the rod 202 {LM) and the level of slurry in the tub level {TL).The free tub level {FTL) is obtained by: FTL = LM-TL. 7

It will be appreciated that the exact form of level sensor is not important to the overall effect of the invention. What is important is to obtain an indication of the variation versus time of the tub slurry volume (called “tub flow” in this document). This can be obtained using a float or a load sensor or combinations of any of these or any other sensor giving this information. 5 The outputs of the flow sensors and level sensors are used to monitor the solid fraction of theslurry in the following manner:

The solid fraction computation is based on the balance between incoming and outgoing volumes(or flow rates) as expressed in the following relationship: ôwater ôcement ~~ ôslurry ôtub 10 where Qtub is the tub rate.

Tub rate is the variation versus time of the tub volume and is considered as positive while the tublevel increases and négative while it decreases. The smaller the tub cross section, the moresensitive the measurement will be to change. Qtub is given by: 15 where Sz„z, is the tub cross section and dh, dt tub is the tub level variation over time. In the simplest case, the tub section is constant and the tub rate becomes the product of the tub levelvariation/time and the tub cross section.

The solids fraction at time t is computed as the ratio of (slurry vol - water vol) over the totalslurry volume présent at time t in the tub. The variation in tub slurry volume Viub(t + <5?)- Vlub(t) 20 can be expressed as: ν,Λ (t + St)- V,b (z) = [β„„ (z)+β„.„ (z) - β.,„, (z)J « Stwhich can be rewritten as: ^(<+«)-^(0=c». W»· U M b *· 8 0124 15

In the same way, the variation in the water volume présent in the tub at time t

Vwaler 0 + <0 ~ ^wa,er (0 is equal to the incoming water volume minus the amount of water présentin the slurry leaving the tub, and can be expressed as: {t + δί}- Vwoter(t) = [Qwater0-(1 ~ SolidFraction{t))* Qslurry (/)]*δί.

Solid Fraction is then expressed as:

SolidFraclion(t + <0 = 1-

Vvater 0 + 0 “ O ~ SolidFractionÇt)) * Qdurry 0J * δί rJfï+QjtYa

The calculation requires that the initial conditions be known if it is to be accurate ab initio, i.e. isthe tub empty, full of water or containing slurry already. The calculation will ultimately stabiliseindependently of the initial conditions, the time taken to do this depending on the tub volume and the output flow rate Qsiurry·

These calculations are conveniently performed using a computer, in which case themeasurements can be provided directly from the sensors via a suitable interface. A preferredscreen display will show the various flow rates or levels, together with the desired solids fraction(calculated when designing the slurry). The mixing process is controlled by adjusting the amountof cernent and/or water added to the mixer so as to maintain the calculated solids fraction at thedesired level. Altematively, the results of the calculations can be fed to an automatic controlSystem which adjusts the rate at which the components are delivered to the mixing System.

The System described above Works well when the dry ingrédients (blend of cernent + additives)are delivered pre-mixed to the well site from another location. In this case essentially the samemeasurements and calculations as described above are performed, merely substituting Qbiend forQcement- If it is desired to mix the dry materials on site as part of the continuous mixing process, aslightly different approach is required. Figure 6 shows a mixing System according to anotherembodiment of the invention and uses a numbering scheme which foliows that of Figure 2. TheSystem of Figure 6 comprises an additional dry material supply 130 which admits the dryproducts to the mixing System 106 via a mass flow meter 132 (other flow measurement meanscan also be used) and a control valve 134. In this case, the basic control équation becomes:

9 012415 where four of the five variables are know and Qcement is the most difficult parameter to measureaccurately. Where multiple dry additives are to be added, the supply can comprise separatematerial supplies, each with a flow meter and valve. Additional terms Qadditivei, Qadditive2, etc., areincluded in the control équation.

It will be appreciated that changes can be made in implémentation while still remaining withinthe scope of using solid fraction as the property monitored to effect control of the mixing.

For example, the method can be applied to the mixing of other borehole fluids such asstimulation fluids (fracturing fluids) or even drilling fluids (mud). In the case of fracturing fluids,the gel and proppant (liquid and solid phases) are usually mixed using a pod blender and theproportion of gel and proppant controlled using a densitometer (usually radioactive) downstreamof the mixer/blender. The use of radioactive sensors generates many environmental issues andwhile Coriolis-type meters are an alternative, they are know to hâve limitations in respect of flowrate when used this way. The présent invention allows control of proppant and gel concentrationsby means of flow meters without the need to rely on densitometer measurements.

Gel and mixed fluid flow rates are measured by means of electromagnetic flow meters. Theamount of proppant is directly deduced from the folio wing relationship: Ôgel + Qproppant QhiixedFluid

Proppant concentration (in Pounds Per Gallon Added or “PPA”) can be a function of solidfraction as defined above and expressed as the following: PPA=Proppant Density*Solid Fr action/(1-Solid Fraction).

Thus the solid fraction measurement methodology described above in relation to cernent can beapplied to fracturing fluids by determining proppant density rather than cernent density.

This approach has the advantage of not requiring the use of radioactive densitometers thusavoiding limitations placed on use for regulatory reasons and without the flow rate performancelimitations of other measurement techniques. The equipment and control System is essentially thesame as that used in the cementing System described above.

Claims

10 012415 CLAIMS 10 15 20 2 A System for mixing a cernent slurry comprising: i) a liquid material supply (101) including a device (104) for measuring the amountof liquid material supplied; ii) a solid cernent supply ( 108); iii) a mixer including a tub (114) which receives the liquid and solid cernent andincludes an output (124) for delivering materials from the mixer to a deliverysystem; iv) a flow meter ( 128) in the output (124); and v) a computing device, characterised in that the system further comprises a device (120) for measuring the amount of materialin the mixer; the computing device receives the output from the device (120) for measuring theamount of material in the mixer and from the flow meters (104, 128) and isarranged to calculate the variation in the amount of material in the tub (114) overtime, and the solid fraction of the mixture; and the calculated solid fraction is used to control the relative amounts of solid andliquid material added to the mixer. A system as claimed in claim 1, wherein the flow meters are selected from mass flowmeters and volumétrie flow meters. A system as claimed in any preceding claim , wherein the flow meters are selected fromCoriolis meters and electromagnetic meters. 11 ο 12415 4 A System as claimed in any preceding claim, wherein the mixer comprises a mixingsection and the tub, mixed materials being fed from the mixing section to the mixing tub,and a portion of the materials in the tub being recirculated to the mixing section. 5 A System as claimed in claim 4, wherein the device comprises a level sensor in the tub. 5 6 A System as claimed in claim 4, wherein the device comprises a load sensor which measures the weight of the tub. 7 A System as claimed in any of claims-4 - 8, wherein recirculation of material takes placeupstream of the flow meter in the output. 8 A System as claimed in any preceding claim, comprising separate supplies of cernent and 10 dry additives, a flow meter being provided to measure the rate of flow of the dry additives. 9 A System as claimed in claim 8, wherein the supply of dry additives comprises multipleseparate supplies of additives, each with its own flow meter. 10 A System as claimed in any preceding claim, wherein the liquid supply includes at least 15 one tank. 11 A System as claimed in claim 10, wherein the device for measuring the amount of liquidsupplied comprises a level sensor in the tank or a flow meter which measures the amountof liquid flowing from the tank. 12 012415 12 A method of mixing a cernent slurry, wherein the cernent and liquid components arecontinuously delivered to a mixer and the cernent slurry is continuously removed fromthe mixer for use, the method comprising: i) measuring the flow rate of liquid components into the mixer; and 5 ii) measuring the flow rate of slurry removed ffom the mixer; characterised by iii) measuring the amount of slurry in the mixer; iv) using the measurements of flow rate and amount of slurry to calculate the solidfraction of the fluid; and 10 (v) controlling the delivery of cernent and/or liquid components to the mixer according to the calculated solid fraction. 13 A method as claimed in claim 12, wherein additives are delivered to the mixer separatelyfrom cernent, the method further comprising measuring the flow rate of the additives delivered to the mixer. 15 14 A method as claimed in claim 12 or 13 wherein the mixer includes a tub, the measurement of the amount of slurry in the mixer comprising a measurement of the amount of slurry in the tub. 15 A method as claimed in claim 14, wherein a portion of the slurry in the tub is recirculatedinto the mixer as solid and liquid components are added. 20 16 A method as claimed in claim 15, wherein the recirculation takes place upstream of the measurement of the flow rate of slurry removed from the mixer. 17 A method as claimed in any of daims 12 -16, wherein liquid components are delivered for mixing from a liquid supply including at least one tank, the method further including the steps of measuring the level of liquid in the tank over time and computing 25 displacement volume as: Σ (V(hln) - V(h2n)) 13 012415 where: V(h) is the exact volume of the tank at level (h) hln is the start level of the n th displaced tank volume; and h2n is the stop level of the n th displaced tank volume.